The structure, chemical characteristics, and biologic function of purified components of the basement membrane were studied by rotary shadowing electron microscopy. Purified protease-derived fragments of laminin, type IV collagen, fibronectin, type V collagen and basement membrane proteoglycan were studied for a) biologic function, b) binding properties with other basement membrane molecules, and c) physical and chemical properties. A model was proposed for the structure of laminin and type IV collagen and which domains of these molecules mediate various biologic functions. The carbohydrate domains on the laminin molecule were mapped. The location of the type IV collagen, laminin receptor, and proteoglycan binding domains on laminin was determined. A model was proposed for the chemical and structural organization of the basement membrane.